ABSTRACT We have been interested in understanding novel determinants of macrophage activation as therapeutics tools in manipulating lung inflammation and its resolution. We identified MEK pathways as novel suppressors of macrophage reparative function.By targeting both MEK1 and MEK2 (MEK1/2) using commercially available inhibitors, we have been able to enhance the ability of macrophages to clear apoptotic PMNs and respond to IL- 4. Importantly, we showed a therapeutic benefit of MEK1/2 inhibition in 2 murine lung injury models: sterile acute lung injury (ALI) using LPS, and bacterial pneumonia using Pseudomonas aeruginosa. In these ALI models, mice treated with MEK inhibitors at 24 and 72 h post-LPS or bacteria exposure experienced improved activity, faster recovery of body weight, reduced pulmonary neutrophilia, and greater M2 polarization.To address the mechanism and cell source driving this improvement in outcome, we generated mice deficient in MEK1 in myeloid cells (LysMCre+MEK1fl). These mice have no phenotype in nave conditions, but experience 100% mortality with LPS-induced ALI using a moderate LPS dose from which all wild-type mice recover. These mice have a similar early inflammatory response to LPS, but fail to turn off inflammation at later time-points. Interestingly, this phenotype can be completely rescued with MEK1/2 inhibitors given at days 1 and 3, suggesting that MEK2 is an important contributor to non-resolving ALI.Importantly, we also identified a SNP in MEK2 associated with death in ARDS patients, suggesting that MEK pathways may be clinically significant in ARDS. These results highlight a potentially novel mechanism of inflammatory control in the macrophage with striking consequences in murine lung injury models. The proposed aims below outline our approach to identify how MEK1 and MEK2 work in concert to regulate macrophage responses to LPS to better define a novel regulatory role of both MEK pathways and macrophages in ALI resolution. In aim 1, we will determine role and cell source of MEK1 in controlling lung inflammation and test our hypothesis that loss of MEK1 in myeloid cells leads to dysregulated inflammatory signals from the myeloid compartment in LPS-induced ALI and contributes to enhanced inflammatory cell recruitment to the lung. In aim 2a, we will determine role of MEK2 in acute lung inflammation and its resolution and test our hypothesis that absence of MEK1 leads to sustained MEK2 activity as the driver of prolonged inflammatory responses to LPS.!In aim 2b, we plan to evaluate MEK1 and MEK2 interactions and mechanisms of MEK2 deactivation in macrophages. These studies would advance our understanding of how the immune system stops inflammation, revealing new therapeutic targets and approaches that could be brought to the bedside.